In connection with vehicles which are operated with the aid of combustion engines, there is a general requirement for low discharges of harmful substances in the exhaust gases from the engine. These substances principally consist of compounds in the form of nitrogen oxide compounds (NOx), hydrocarbon compounds (HC) and carbon monoxide (CO). According to what is known, the exhaust gases from a conventional petrol engine can be purified using an exhaust gas catalyst which forms part of the exhaust gas system and to which the exhaust gases are conducted. In an exhaust gas catalyst of the three-way catalyst type, the greater part of the abovementioned harmful compounds are eliminated by means of known catalytic reactions. In order to optimize the function of the catalyst, so as to ensure that it affords as large a purifying effect as possible as far as NOx, HC and CO compounds are concerned, the engine is operated, in most operational cases, using a stoichiometric air/fuel mixture, i.e. a mixture in which λ=1. A computer-based control unit can be used to ensure that this stoichiometric mixture is maintained in the operational cases which may be relevant for each respective vehicle.
Furthermore, there is a general requirement, in connection with vehicles, to reduce the fuel consumption of a combustion engine to the greatest extent possible. To this end, engines possessing new types of combustion chambers in the cylinders of the engine have been developed in recent years, in particular for the purpose of being able to operate the engine using increasingly lean fuel mixtures, i.e. in which λ>1. In an engine of what is termed the lean-burn type (i.e. a directly injected Otto engine), each combustion chamber in the engine can be arranged such that the fuel which is supplied can be concentrated to a high degree at each respective ignition plug. This operational state is normally termed “stratified” operation and, when the engine is run continuously at low or medium torque and rotational speed, permits operation using a very lean air-fuel mixture, more specifically up to approximately λ=3. This results in a substantial economy in the fuel consumption in the case of this type of engine. The engine can also be operated in an additional, “homogeneous” operational state, either using a mixture which is in the main stoichiometric (λ=1) or a mixture which is relatively rich (λ<1). This latter operational state normally exists in situations in which the engine is being run at relatively high torques and rotational speeds. Certain additional states, apart from the stratified state and the homogeneous state, can also be met with in the case of a lean-burn engine.
In lean-burn engines, a conventional three-way catalyst cannot be used to reduce NOx compounds due to the fact that the catalyst is designed to have an optimum purifying ability when the mixture is stoichiometric. For this reason, an ordinary three-way catalyst can be combined with a nitrogen oxide adsorbent (also termed NOx adsorbent or “NOx trap”), which is a device, which is known per se, for absorbing NOx compounds, for example in the exhaust gases from a combustion engine. The NOx adsorbent can in this way be used as a complement to a conventional three-way catalyst, either as a separate unit upstream of the three-way catalyst or integrated with the three-way catalyst, i.e. together with the catalytic material belonging to the three-way catalyst. In the latter case, an integrated component is then constituted in the form of an NOx-adsorbing exhaust gas catalyst.
It may be noted that the requirement with regard to reduction of NOx compounds is relevant in relation to different types of combustion engines, for example in the case of diesel engines as well as lean-burn engines as mentioned above. In such situations, therefore, it is not possible for a conventional three-way catalyst to function as a catalyst for reduction of NOx compounds.
An NOx adsorbent is constituted such that it takes up (adsorbs) NOx compounds in the exhaust gases if the engine is being operated with a lean air/fuel mixture and emits (desorbs) the NOx compounds If the engine is run, for a certain period of time, with a rich air/fuel mixture. Furthermore, the NOx adsorbent has the property that it can only absorb NOx compounds up to a certain limit; i.e., it is gradually “filled” and in this way reaches a limit to the adsorption. In this situation, the NOx adsorbent has to be regenerated; i.e., it has to be made to desorb and consequently release the stored NOx compounds. If, then, a conventional three-way catalyst is arranged downstream of an NOx adsorbent, or if, alternatively, a three-way catalyst is designed such that it is integrated with an NOx adsorbent, the desorbed NOx compounds can be eliminated by the three-way catalyst provided the latter has reached its ignition temperature.
According to the prior art, an NOx adsorbent can be regenerated by means of the exhaust gas mixture which is flowing through the NOx adsorbent being made relatively rich for a certain period of time, of the order of size of a few seconds. In practice, this is done by the engine being operated in the abovementioned homogeneous operational state for this period of time; i.e., with the engine consequently being operated with a relatively rich air/fuel mixture. In this way, the NOx adsorbent is “emptied” such that it can subsequently adsorb NOx compounds for a certain time which continues until a new regeneration becomes necessary.
Although the abovementioned procedure for regenerating NOx compounds functions well per se, it suffers from a disadvantage in that it negatively effects the fuel consumption of the engine because it is based on the engine having to be operated with a rich exhaust gas mixture at certain intervals. There is consequently a need for alternative methods for achieving an efficient reduction of NOx compounds in exhaust gases having a relatively large excess of oxygen, as is the case with lean-burn engines and diesel engines. A similar situation exists in the case of other combustion processes involving NOx compounds in an excess of oxygen, for example in connection with incineration plants, combined power and heating plants, residential boilers, flue gas purification and the like.